Im assuming RO is actually water treated through a reverse osmosis filter rather than real ocean salt water...
in which case i belive the op has it spot on in there salt levels got scrwede when they mixed the water change water.
op, welcome to the salty world, and dont be too hard on yourself...
good point holly...
if your adding salt water at every water change your going to mass overdo your salt levels...
top offs should be fresh water unless your salinity is low.
remove everything that is dead or looks to be dying...
you can try to save dyeing items by putting them in a hospital tank, they may not survivie but at least in a hopsital tank they wont effect the tank paremeters and you can start trying to save the tank.
if your salinity is too high add...